FIELD
[0001]
The present invention relates to a hot stamped body and a method for producing the same
10 and to an Al-plated steel sheet, more particularly relates to a hot stamped body including Al
plating and excellent in corrosion resistance after coating and a method for producing the same
and to an Al-plated steel sheet suitable for producing the hot stamped body.
BACKGROUND
15 [0002]
As the technique for press-forming a material which is hard to shape such as high strength
steel sheet, hot stamping (hot pressing) is known. Hot stamping is a technique for hot shaping
which shapes a material supplied for shaping after heating it. In this technique, the material is
shaped after being heated, therefore at the time of shaping, the steel material is soft and has
20 excellent shapeability. Therefore, even a high strength steel material can be formed into a
complicated shape with a good precision. Further, it is known that the shaped steel material has
sufficient strength since it is hardened at the same time as being shaped by the press die.
[0003]
In the past, to improve the chemical convertability, coating film adhesion, slidability, etc.,
25 of a plated steel sheet for hot stamping, for example, it is known to provide the plated steel sheet
with a surface treated layer containing ZnO.
[0004]
PTL 1 describes a hot dip zinc-based coated steel sheet provided with a surface treated layer
containing one or more oxides having particle sizes of 5 nm or more and 500 nm or less selected
30 from zirconia, lanthanum oxide, cerium oxide, and neodymium oxide in a range per side of 0.2
g/m2 or more and 2 g/m2 or less. Further, PTL 1 teaches that the phosphate treatability after hot
pressing is raised and the coating film adhesion is improved by the presence of zirconia,
lanthanum oxide, cerium oxide, and neodymium oxide in the surface treated layer at the time of
heating before the hot pressing and by the Al oxides formed at the time of hot pressing being
35 rendered harmless and thereby the promotion of the formation of zinc oxide (ZnO) at the time of
hot pressing.
2
[0005]
PTL 2 describes a surface treatment solution for a plated steel sheet for hot pressing
containing a ZnO aqueous dispersion (A) and an aqueous dispersible organic resin (B) wherein
the ZnO aqueous dispersion (A) contains water and ZnO particles with an average particle size
of 10 to 300 nm, the aqueous dispersible or 5 ganic resin (B) has an emulsion average particle size
of 5 to 300 nm, and a mass ratio (WA /WB ) of a mass (WA ) of ZnO particles in the ZnO
aqueous dispersion and a mass (WB ) of solid content of the aqueous dispersible organic resin is
30/70 to 95/5. Further, PTL 2 describes that by using the above surface treatment solution to
form a surface treatment film containing ZnO particles and an aqueous dispersible organic resin
10 in a specific mass ratio on the surface of a plated steel sheet, it is possible to secure waterproofness
of the film, solvent resistance, and adhesion with the plated steel sheet and obtain a
plated steel sheet stably excellent in hot lubrication ability, chemical conversion ability after hot
pressing, corrosion resistance after coating, and spot weldability.
[0006]
15 PTL 3 describes an Al-based plated steel sheet provided with a steel sheet, an Al-based
plating layer formed on one side or both sides of the steel sheet and containing at least Al in a
mass% of 85% or more, and a surface coating layer laid on the surface of the Al-based plating
layer and containing ZnO and one or more lubrication improving compounds, the lubrication
improving compounds being compounds containing transition metal elements of one or more of
20 any of Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zr, Mo, W, La, and Ce. Further, PTL 3 describes that
according to such an Al-based plated steel sheet, it is possible to acquire better lubrication ability
than the past, possible to realize improvement of the shapeability and productivity at the time of
hot pressing, and furthermore possible to realize even improvement of the chemical conversion
ability and the corrosion resistance after coating after hot pressing.
25 [0007]
In addition to the above patent literature, as literature describing a surface treated layer able
to incorporate ZnO in a plated steel sheet, for example, PTLs 4 to 6 may be mentioned.
[CITATIONS LIST]
30 [PATENT LITERATURE]
[0008]
[PTL 1] WO 2016/159307
[PTL 2] WO 2016/195101
[PTL 3] WO 2013/157522
35 [PTL 4] Japanese Unexamined Patent Publication No. 2013-519793
[PTL 5] Japanese Unexamined Patent Publication No. 2008-127638
3
[PTL 6] Japanese Unexamined Patent Publication No. 2007-514865
SUMMARY
[TECHNICAL PROBLEM]
5 [0009]
For example, in an Al-based plated steel sheet for hot stamping, a plating layer containing
an alloy layer containing Al and Fe and, in some cases, further Si is formed on the steel base
material by Fe diffusing from the steel base material (base iron) due to the hot stamping. It is
known that such an alloy layer is relatively hard, therefore the hot stamped body provided with
10 the plating layer containing the alloy layer is resistant to defects reaching the steel base material
and in general is excellent in corrosion resistance after coating. However, even in such a hot
stamped body, once a defect reaching down to the steel base material is formed, the steel base
material will become increasingly corroded and the corrosion resistance after coating is liable to
fall. For this reason, there is a need for a hot stamped body containing an Al-based plating
15 exhibiting a high corrosion resistance after coating even under more severe conditions and for an
Al-plated steel sheet suitable for producing the hot stamped body.
[0010]
The invention described in PTL 1 relates to hot dip zinc-based coated steel sheet. For this
reason, PTL 1 does not describe or suggest in any way a hot stamped body containing an Al20
based plating and improvement of its corrosion resistance after coating.
[0011]
On the other hand, PTL 2 describes a steel sheet, on which a plating layer containing Al is
formed, as a plated steel sheet and describes that by forming a surface treatment film containing
ZnO particles and an aqueous dispersible organic resin in a specific mass ratio on the surface of
25 such a plated steel sheet, it is possible to obtain a plated steel sheet excellent in corrosion
resistance after coating, etc. However, in PTL 2, the additional constituents contained in the
surface treatment film are not necessarily sufficiently studied. Therefore, in the invention
described in PTL 2, there was still room for improvement in relation to the enhancement of the
corrosion resistance after coating, etc.
30 [0012]
Further, in PTL 3, improvement of the corrosion resistance after coating is specifically
shown in an Al-based plated steel sheet provided with a surface coating layer containing ZnO
and lubrication improving compounds containing Ni, Mn, and other specific transition metal
elements, but the corrosion resistance after coating in the case of use of lubrication improving
35 compounds containing other transition metal elements is not necessarily sufficiently studied.
Therefore, in the invention described in PTL 3, there was still room for improvement in relation
4
to the enhancement of the corrosion resistance after coating, etc.
[0013]
Therefore, the present invention has as its object the use of a novel constitution to provide a
hot stamped body excellent in corrosion resistance after coating including Al plating and a
method for producing the 5 same and an Al-plated steel sheet suitable for producing the hot
stamped body.
[SOLUTION TO PROBLEM]
[0014]
10 The present invention to achieve the above object is as follows:
(1) A hot stamped body comprising
a steel base material,
an Al-plating layer formed on at least one surface of the steel base material,
a coating formed on the Al-plating layer and containing ZnO particles and CeO2 particles
15 having an average particle size smaller than an average particle size of the ZnO particles, and
a Zn- and Al-containing complex oxide layer formed between the Al-plating layer and the
coating.
(2) The hot stamped body according to the above (1), wherein the coating does not contain
an organic binder.
20 (3) The hot stamped body according to the above (1) or (2), wherein the coating has a
structure in which the CeO2 particles are deposited around the ZnO particles.
(4) The hot stamped body according to any one of the above (1) to (3), wherein an amount
of deposition of ZnO in the coating is 0.60 g/m2 or more and 13.00 g/m2 or less.
(5) The hot stamped body according to the above (4), wherein the amount of deposition of
ZnO in the coating is 1.20 g/m2 or more and 10.00 g/m2 25 or less.
(6) The hot stamped body according to any one of the above (1) to (5), wherein the coating
contains the CeO2 particles in 1.0 mass% or more and 30.0 mass% or less with respect to a total
amount of the ZnO particles and the CeO2 particles.
(7) The hot stamped body according to the above (6), wherein the coating contains the
30 CeO2 particles in 2.0 mass% or more and 25.0 mass% or less with respect to the total amount of
the ZnO particles and the CeO2 particles.
(8) The hot stamped body according to any one of the above (1) to (7), wherein an average
particle size of the ZnO particles is 0.003 m or more and 8.000 m or less and an average
particle size of the CeO2 particles is 3.0% or more and 20.0% or less of the average particle size
35 of the ZnO particles.
(9) The hot stamped body according to the above (8), wherein the average particle size of
5
the ZnO particles is 0.010 m or more and 5.000 m or less and the average particle size of the
CeO2 particles is 8.0% or more and 12.5% or less of the average particle size of the ZnO
particles.
(10) The hot stamped body according to the above (9), wherein the average particle size of
the ZnO particles is 0.020 m or more and 4.000 m or le 5 ss and the average particle size of the
CeO2 particles is 9.0% or more and 12.0% or less of the average particle size of the ZnO
particles.
(11) The hot stamped body according to any one of the above (1) to (10), wherein the steel
base material comprises, by mass%,
10 C: 0.01 to 0.50%,
Si: 2.00% or less,
Mn: 0.01 to 3.50%,
P: 0.100% or less,
S: 0.050% or less,
15 Al: 0.001 to 0.100%,
N: 0.020% or less,
Ti: 0 to 0.100%,
B: 0 to 0.0100%,
Cr: 0 to 1.00%,
20 Ni: 0 to 5.00%,
Mo: 0 to 2.000%,
Cu: 0 to 1.000%,
Ca: 0 to 0.1000%, and
a balance of Fe and impurities.
25 (12) The hot stamped body according to any one of the above (1) to (11), wherein the Alplating
layer comprises Si and a balance of Al, Fe and impurities.
(13) A method for producing a hot stamped body according to any one of the above (1) to
(12), comprising
forming an Al-plating layer on at least one side of a steel sheet,
30 coating a surface of the Al-plating layer with an aqueous solution containing ZnO particles
and CeO2 particles, then heating it to form a coating containing ZnO particles and CeO2
particles on the Al-plating layer, and
hot pressing the steel sheet having the formed coating eon.
(14) An Al-plated steel sheet comprising
35 a steel base material,
an Al-plating layer formed on at least one surface of the steel base material, and
6
a coating formed on the Al-plating layer and containing ZnO particles and CeO2 particles
having an average particle size smaller than an average particle size of the ZnO particles.
(15) The Al-plated steel sheet according to the above (14), wherein the coating does not
contain an organic binder.
(16) The Al-plated steel sheet according to the above 5 (14) or (15), wherein the coating has a
structure in which the CeO2 particles are deposited around the ZnO particles.
[ADVANTAGEOUS EFFECTS OF INVENTION]
[0015]
10 According to the present invention, by using an Al-plated steel sheet comprising an Alplating
layer and a coating formed thereon and containing ZnO particles and having added to the
coating CeO2 particles as a corrosion inhibitor and by hot stamping the Al-plated steel sheet,
even if a defect reaching down to the steel base material is formed at the obtained hot stamped
body, Ce is made to be eluted from the CeO2 particles at the defect part and can form a
15 protective coating at the cathodic reaction region of the exposed part of the steel base material,
therefore the progression of the cathodic reaction at the exposed part of the steel base material
can be suppressed. As a result, according to the present invention, it is possible to obtain a hot
stamped body remarkably suppressed in occurrence of coating film blisters, etc., and excellent in
corrosion resistance after coating, in particular long term corrosion resistance after coating.
20
BRIEF DESCRIPTION OF DRAWINGS
[0016]
FIG. 1 is a schematic view showing one side part of a hot stamped body of the present
invention.
25 FIGS. 2A to 2C show images of a cross-section of a coating obtained by observation using a
scan electron microscope (SEM), wherein FIG. 2A shows a cross-sectional SEM image of a
coating containing only ZnO particles and not containing CeO2 particles (Comparative Example
45), FIG. 2B shows a cross-sectional SEM image of a coating containing ZnO particles and
CeO2 particles (CeO2 particle content: 5 mass%), and FIG. 2C shows an enlarged view of FIG.
30 2B.
FIG. 3 is a view schematically showing a method of measurement of an average particle
size relating to ZnO particles and CeO2 particles in the present invention.
FIG. 4 shows a cross-sectional image obtained by an SEM of a hot stamped body according
to one embodiment of the present invention (amount of deposition of ZnO: 2.49 g/m2).
35
DESCRIPTION OF EMBODIMENTS
7
[0017]

The hot stamped body of the present invention comprises
a steel base material,
an Al-plating layer formed on a 5 t least one surface of the steel base material,
a coating formed on the Al-plating layer and containing ZnO particles and CeO2 particles
having an average particle size smaller than an average particle size of the ZnO particles, and
a Zn- and Al-containing complex oxide layer formed between the Al-plating layer and the
coating.
10 [0018]
In an Al-based plated steel sheet for hot stamping, first, in general, a plating layer
containing Al and Si is formed on a steel base material (base iron). By performing hot stamping,
Fe diffuses from the steel base material whereby a plating layer containing an alloy layer made
of Al-Fe-Si, etc., is formed on the steel base material. As explained above, it is known that such
15 an alloy layer is relatively hard, therefore the hot stamped body provided with the plated layer
containing the alloy layer is resistant to defects reaching the steel base material and in general is
excellent in corrosion resistance after coating. However, even in such a hot stamped body, once a
defect reaching down to the steel base material is formed, the steel base material will become
increasingly corroded and the corrosion resistance after coating is liable to fall.
20 [0019]
More specifically, if a defect reaching down to the steel base material is formed at a hot
stamped body including Al plating, plating corrosion where the exposed part of the steel base
material acts as the cathode and the Al-plated part acts as the anode will occur at the interface of
the steel base material and the plating. In addition, at the exposed part of the steel base material,
the cathodic reaction of the dissolved oxygen (O2 +2H2 O+4e- 4OH- 25 ) will proceed fast,
therefore in relation to this, sometimes corrosion at the interface of the steel base material and
plating will progress and coating film blisters and other phenomena will arise.
[0020]
Therefore, the inventors studied the addition of a corrosion inhibitor to a coating provided at
30 a hot stamped body including Al plating for improving the chemical convertability, coating
adhesion, slidability, etc., more specifically a coating containing zinc oxide (ZnO) particles. On
the other hand, in selecting the corrosion inhibitor, in addition to the corrosion prevention action
such as the above which is mainly focused on, from a practical viewpoint, the solution stability
at the time of industrial production (nonoccurrence of precipitation, etc.) is extremely important.
35 The inventors discovered that when not considering solution stability, several materials such as
boric acid exhibit a suitable corrosion prevention action, but the inventors further studied various
8
materials as corrosion inhibitors able to realize both a corrosion prevention action and solution
stability and as a result discovered that it is effective to use cerium oxide (CeO2 ) particles as the
corrosion inhibitor.
[0021]
Explained in more detail, the inventors discovered that 5 by using an Al-plated steel sheet
comprising an Al-plating layer and a coating formed thereon and containing ZnO particles and
having added to the coating CeO2 particles having an average particle size smaller than the
average particle size of the ZnO particles as a corrosion inhibitor and by hot stamping the Alplated
steel sheet, even if a defect reaching down to the steel base material is formed at the
10 obtained hot stamped body, Ce is made to be eluted from the CeO2 particles at the defect part
and can form a protective coating at the cathodic reaction region of the exposed part of the steel
base material, therefore the progression of the cathodic reaction at the exposed part of the steel
base material can be suppressed. As a result, the inventors discovered that it is possible to obtain
a hot stamped body remarkably suppressed in occurrence of coating film blisters, etc., and
15 excellent in corrosion resistance after coating, in particular long term corrosion resistance after
coating.
[0022]
While not intending to be constrained to any specific theory, it is believed that at the above
defect part, Ce4+ ions are eluted from the CeO2 particles. Further, it is believed that the eluted
Ce4+ 20 ions move toward the exposed part of the steel base material which has become an alkali
environment due to progression of the cathodic reaction (O2 +2H2 O+4e- 4OH- ) so as to
maintain electrical neutrality. Here, Ce4 + ions are stably present in an alkali environment in the
form of hydroxides, therefore precipitate as cerium hydroxide Ce(OH)4 . Further, it is believed
that this precipitated coating acts as a protective film and inhibits further progression of the
25 cathodic reaction at the exposed part of the steel base material. Further, it is believed that by
using CeO2 particles having an average particle size smaller than the average particle size of the
ZnO particles, it is possible to make the ZnO particles aggregate relatively densely with each
other and reduce the space between ZnO particles, therefore for example it is possible to make
the elution of Ce from the CeO2 particles deposited around these particles proceed relatedly
30 slowly and as a result it becomes possible to achieve long term corrosion resistance after coating.
[0023]
[Steel Base Material]
The steel base material according to a hot stamped body of the present invention may be
any steel material having a thickness and composition generally used in hot stamped bodies. As
35 such a steel base material, while not particularly limited to this, a steel material having a 0.3 to
2.3 mm thickness and/or containing a chemical composition comprised of, by mass%, C: 0.01 to
9
0.50%, Si: 2.00% or less, Mn: 0.01 to 3.50%, P: 0.100% or less, S: 0.050% or less, Al: 0.001 to
0.100%, and N: 0.020% or less and a balance of Fe and impurities can be mentioned. Below, the
constituents contained in the above steel base material which is preferably applied in the present
invention will be explained in more detail. In the following explanation, the “%” relating to the
contents of the constituents m 5 eans “mass%” unless otherwise indicated.
[0024]
[C: 0.01 to 0.50%]
C is an element unavoidably contained in steel and/or included so as to secure the targeted
mechanical strength. Excessively reducing the C content causes the refining costs to increase,
10 therefore the C content is preferably 0.01% or more. Further, if the C content is less than 0.10%,
a need arises to include other alloy elements in large amounts so as to secure the mechanical
strength, therefore from the viewpoint of securing the mechanical strength, the C content is
preferably 0.10% or more or 0.20% or more. On the other hand, if including C in more than
0.50%, the steel material can be made further harder, but becomes brittle and sometimes hot dip
15 cracks form. Therefore, the C content is preferably 0.50% or less. From the viewpoint of
preventing hot dip cracks, the content is more preferably 0.40% or less or 0.30% or less.
[0025]
[Si: 2.00% or Less]
Si is an element which is added as a deoxidizer and otherwise unavoidably included in the
20 refining process of steel and an element having the effect of improving strength. The Si content
may be 0%, but from the viewpoint of improving strength, it is preferably 0.01% or more. For
example, Si content may be 0.05% or more or 0.10% or more. On the other hand, excessive
inclusion of Si sometimes causes a drop in ductility in the hot rolling step at the time of
production of steel sheet or as a result causes deterioration of the surface properties. For this
25 reason, the Si content is preferably 2.00% or less. For example, the Si content may be 1.50% or
less or 1.00% or less. Further, Si is an easily oxidizable element. It forms an oxide film on the
steel sheet surface, therefore if the Si content is more than 0.60%, at the time of hot dip coating,
there is a possibility of the wettability falling and nonplating defects arising. Therefore, more
preferably, the Si content is 0.60% or less.
30 [0026]
[Mn: 0.01 to 3.50%]
Mn also, like Si, is an element which is added as a deoxidizer and otherwise unavoidably
included in the refining process of steel, but has the effects of improving the strength and
improving the hardenability and, furthermore, has the effect of suppressing the hot embrittlement
35 due to S. Inclusion of 0.01% or more is preferable. For example, the Mn content may be 0.10%
or more or 0.50% or more. On the other hand, if excessively including Mn, sometimes
10
deterioration of the uniformity of quality due to segregation at the time of casting, excessive
hardening of the steel, and a drop in the ductility at the time of hot and cold working are invited,
therefore the Mn content is preferably 3.50% or less. For example, the Mn content may be 3.00%
or less or 2.00% or less.
5 [0027]
[P: 0.100% or Less]
P is an unavoidably contained element, but is also a solution strengthening element and is
an element which can improve the strength of a steel material at relatively low cost. However,
excessive inclusion of P sometimes invites a drop in toughness, therefore the P content is
10 preferably 0.100% or less. For example, the P content may be 0.050% or less or 0.020% or less.
On the other hand, the lower limit of the P content may be 0%, but from the refining limit is
preferably 0.001%. For example, the P content may be 0.003% or more or 0.005% or more.
[0028]
[S: 0.050% or Less]
15 S is also an unavoidably included element. It forms inclusions as MnS which act as starting
points for breakage and sometimes harms ductility and toughness and becomes a cause of
inferior workability. For this reason, the lower the S content, the more preferable. 0.050% or less
is more preferable. For example, the S content may be 0.020% or less or 0.010% or less. On the
other hand, the lower limit of the S content may be 0%, but from the cost of production is
20 preferably 0.001%. For example, the S content may be 0.002% or more or 0.003% or more.
[0029]
[Al: 0.001 to 0.100%]
Al is an element used as a deoxidizer at the time of steelmaking. From the refining limit, the
lower limit of the Al content is preferably 0.001%. For example, the Al content may be 0.005%
25 or more or 0.010% or more. Further, Al is an element obstructing plateability, therefore the
upper limit of the Al content is preferably 0.100%. For example, the Al content may be 0.080%
or less or 0.050% or less.
[0030]
[N: 0.020% or Less]
30 N is also an unavoidably included element. However, if too great, an increase in the
production costs may be expected, therefore the upper limit of the N content is preferably
0.020%. For example, the N content may be 0.015% or less or 0.010% or less. On the other
hand, the lower limit of the N content may be 0%, but from the cost in production is preferably
0.001%. For example, the N content may be 0.002% or more or 0.003% or more.
35 [0031]
The basic chemical composition of the steel base material suitable for use in the present
11
invention is as explained above. Furthermore, the steel base material may optionally contain one
or more of Ti: 0 to 0.100%, B: 0 to 0.0100%, Cr: 0 to 1.00%, Ni: 0 to 5.00%, Mo: 0 to 2.000%,
Cu: 0 to 1.000%, and Ca: 0 to 0.1000%. Below, these optional elements will be explained in
detail.
5 [0032]
[Ti: 0 to 0.100%]
Ti is one of the strengthening elements and an element improving the heat resistance of the
Al-based plating layer. If the Ti content is less than 0.005%, the effect of improvement of the
strength or the heat resistance cannot be sufficiently obtained, therefore the Ti content is
10 preferably 0.005% or more. For example, Ti content may be 0.010% or more or 0.015% or more.
On the other hand, if excessively including Ti, for example, carbides and nitrides are formed
leading to softening of the steel material, therefore the Ti content is preferably 0.100% or less.
For example, the Ti content may be 0.080% or less or 0.050% or less.
[0033]
15 [B: 0 to 0.0100%]
B is an element having the effect of acting to improve the strength of the steel material at
the time of hardening. If the B content is less than 0.0003%, such an effect of improvement of
the strength is not sufficiently obtained. On the other hand, if more than 0.0100%, inclusions (for
example, BN, borocarbides, etc.) are formed resulting in embrittlement and the fatigue strength
20 is liable to be lowered. Therefore, the B content is preferably 0.0003 to 0.0100%. For example,
the B content may be 0.0010% or more or 0.0020% or more and/or 0.0080% or less or 0.0060%
or less.
[0034]
[Cr: 0 to 1.00%]
25 Cr has the effect of suppressing the formation of nitrides formed at the interface of the Albased
plating layer and causing the Al-based plating layer to peel off. Further, Cr is also an
element improving the wear resistance and improving the hardenability. If the Cr content is less
than 0.01%, the above effects cannot be sufficiently obtained. On the other hand, if the Cr
content is more than 1.00%, not only do the above effects become saturated, but also the
30 production costs of the steel material rise. Therefore, the Cr content is preferably 0.01 to 1.00%.
For example, Cr content may be 0.05% or more or 0.10% or more and/or 0.80% or less or 0.50%
or less.
[0035]
[Ni: 0 to 5.00%]
35 Ni has the effect of improving the hardenability at the time of hot pressing and also has the
effect of raising the corrosion resistance of the steel material itself. If the Ni content is less than
12
0.01%, these effects cannot be sufficiently obtained. On the other hand, if the Ni content is more
than 5.00%, not only do the above effects become saturated, but also the production costs of the
steel material rise. Therefore, the Ni content is preferably 0.01 to 5.00%. For example, the Ni
content may be 0.05% or more or 0.10% or more and/or 3.00% or less or 2.00% or less.
5 [0036]
[Mo: 0 to 2.000%]
Mo has the effect of improving the hardenability at the time of hot pressing and also has the
effect of raising the corrosion resistance of the steel material itself. If the Mo content is less than
0.005%, these effects cannot be sufficiently obtained. On the other hand, if the Mo content is
10 more than 2.000%, not only do the above effects become saturated, but also the production costs
of the steel material rise. Therefore, the Mo content is preferably 0.005 to 2.000%. For example,
the Mo content may be 0.010% or more or 0.100% or more and/or 1.500% or less or 1.000% or
less.
[0037]
15 [Cu: 0 to 1.000%]
Cu has the effect of improving the hardenability at the time of hot pressing and also has the
effect of raising the corrosion resistance of the steel material itself. If the Cu content is less than
0.005%, these effects cannot be sufficiently obtained. On the other hand, if the Cu content is
more than 1.000%, not only do the above effects become saturated, but also the production costs
20 of the steel material rise. Therefore, the Cu content is preferably 0.005 to 1.000%. For example,
the Cu content may be 0.010% or more or 0.050% or more and/or 0.500% or less or 0.200% or
less.
[0038]
[Ca: 0 to 0.1000%]
25 Ca is an element for controlling inclusions. If the Ca content is less than 0.0002%, the effect
cannot be sufficiently obtained, therefore the Ca content is preferably 0.0002% or more. For
example, Ca content may be 0.0010% or more or 0.0020% or more. On the other hand, if the Ca
content is more than 0.1000%, the alloy cost becomes higher, therefore the Ca content is
preferably 0.1000% or less. For example, the Ca content may be 0.0500% or less or 0.0100% or
30 less.
[0039]
Furthermore, the steel base material according to a hot stamped body of the present
invention may suitably contain other elements in a range not detracting from the effect of the
present invention explained in this Description in addition to or in place of the above optional
35 elements. For example, W, V, Nb, Sb, and other elements may be suitably included.
[0040]
13
In the steel base material according to a hot stamped body of the present invention, the
balance other than the above constituents consists of Fe and impurities. Here, the impurities in
the steel base material are constituents, etc., entering due to various factors in the production
process such as the ore, scrap or other raw materials when industrially producing the hot stamped
5 body according to the present invention.
[0041]
[Al-Plating Layer]
According to the present invention, an Al-plating layer is formed on at least one side of the
steel base material, i.e., on one side or both sides of the steel base material. In the hot stamped
10 body of the present invention, the “Al-plating layer” means a plating layer in which the chemical
composition right after plating mainly consists of Al, more specifically, a plating layer in which
the chemical composition right after plating comprises more than 50 mass% of Al. If performing
hot stamping, Fe diffuses from the steel base material to inside the Al-plating layer, therefore the
chemical composition of the Al-plating layer changes depending on the heat treatment conditions
15 (heating temperature, holding time, etc.) at the time of hot stamping. For example, if the amount
of diffusion of Fe from the steel base material to the Al-plating layer becomes greater, even if the
Al content right after plating is more than 50 mass%, the Al content after hot stamping will fall
from that. Therefore, the chemical composition of the Al-plating layer according to the present
invention after hot stamping does not necessarily have to be Al: more than 50 mass%.
20 [0042]
The Al-plating layer according to the hot stamped body of the present invention preferably
contains Si. In general, it is known that, in an Al-based plated steel sheet for hot stamping, at the
time of the plating treatment, Fe diffuses from the steel base material and that the diffused Fe
reacts with the Al in the plating layer whereby an Al-Fe alloy layer is formed at the interface of
25 the plating layer and the steel base material. An Al-Fe alloy layer is a hard layer, therefore if an
Al-Fe alloy layer is excessively formed, for example, the shapeability of the steel sheet at the
time of cold working is liable to be impaired. Here, it is known that the Si in the Al-plating layer
has the function of suppressing the formation of such an Al-Fe alloy layer. Further, by including
Si, the Al-plating layer according to the present invention after hot stamping is alloyed with the
30 Fe diffusing from the steel base material into the Al-plating layer at the time of hot stamping,
whereby a relatively hard Al-Fe-Si alloy layer can be included. As a result, in the finally
obtained Al-plated hot stamped body, it becomes possible to reliably maintain a high resistance
even against formation of defects which would reach the steel base material.
[0043]
35 Furthermore, the Al-plating layer according to the hot stamped body of the present
invention may also suitably contain other elements in a range not obstructing the effect of the
14
present invention. For example, the Al-plating layer may, in addition to Si, optionally contain
Mg, Ca, Sr, mischmetal, or other elements for improving the corrosion resistance of the Alplating
layer.
[0044]
In the Al-plating layer according 5 to the hot stamped body of the present invention, the
balance other than the above constituents (i.e., Si, Mg, Ca, Sr, mischmetal, and other elements)
consists of Al, Fe and impurities. Here, the “impurities” in the Al-plating layer are raw materials
first and foremost and other constituents, etc., entering due to various factors in the production
process when producing an Al-plating layer (however, excluding Fe eluted from the steel base
10 material into the plating bath and Fe diffusing from the steel base material into the Al-plating
layer at the time of hot stamping).
[0045]
[Coating Containing ZnO Particles and CeO2 Particles Having Average Particle Size Smaller
Than Average Particle Size of ZnO Particles]
15 According to the present invention, the above Al-plating layer is formed with a coating
containing ZnO particles and CeO2 particles having an average particle size smaller than the
average particle size of the ZnO particles. FIG. 1 is a schematic view showing one side part of
the hot stamped body of the present invention (i.e., the hot stamped body of the present invention
may also have an Al-plating layer not only on one side of the steel base material, but both sides).
20 Referring to FIG. 1, it will be understood that the hot stamped body 10 of the present invention
has a structure where an Al-plating layer 2 is formed on one side of a steel base material (base
iron) 1, a coating 3 containing ZnO particles and CeO2 particles is further formed on the Alplating
layer 2, and, while explained in detail later, a Zn- and Al-containing complex oxide layer
6 is formed between the Al-plating layer 2 and coating 3. In general, it is known that by forming
25 the coating containing ZnO particles on the Al-plating layer, the chemical convertability, coating
film adhesion, slidability, etc., of the steel material including the Al plating can be improved.
However, as explained earlier, if a defect reaching down to the steel base material is formed at
such a steel material, plating corrosion where the exposed part of the steel base material acts as
the cathode and the Al-plated part acts as the anode occurs at the interface of the steel base
30 material and the plating. In addition, at the exposed part of the steel base material, the cathodic
reaction of the dissolved oxygen (O2 +2H2 O+4e- 4OH- ) proceeds fast, therefore in relation to
this, sometimes corrosion at the interface of the steel base material and plating progresses and
coating film blisters and other phenomena arise.
[0046]
35 As opposed to this, in the present invention, by further including CeO2 particles having an
average particle size smaller than the average particle size of the ZnO particles in a conventional
15
coating including ZnO particles, even if a defect reaching the steel base material is formed in the
hot stamped body including the Al plating, at the defect part, the cerium constituent, more
specifically, Ce4+ ions, is made to be eluted from the CeO2 particles to form a protective
coating, more specifically a protective coating comprised of Ce(OH)4, at the cathodic reaction
region of the exposed part of the 5 steel base material, therefore it becomes possible to suppress
the further progression of a cathodic reaction at the exposed part of the steel base material. As a
result, according to the present invention, it is possible to obtain a hot stamped body remarkably
suppressed in occurrence of coating film blisters, etc., and excellent in corrosion resistance after
coating.
10 [0047]
In the present invention, it is important that the coating include cerium in the form of CeO2
particles. For example, if the coating includes cerium in the form of a cerium salt of cerium
nitrate (Ce(NO3)3), etc., the elution rate of cerium ions to the exposed part of the steel base
material becomes faster, therefore excessive formation of the protective coating is caused and in
15 turn early depletion of the source of supply of the protective coating (i.e., cerium salts) is invited.
On the other hand, if the coating includes cerium in the form of CeO2 particles, compared with a
cerium salt, the elution rate of cerium ions to the exposed part of the steel base material can be
controlled to a suitable range, therefore this is extremely effective from the viewpoint of
achievement of long term corrosion resistance after coating. Furthermore, it is believed that by
20 using CeO2 particles having an average particle size smaller than the average particle size of the
ZnO particles, it is possible to make the ZnO particles aggregate relatively densely with each
other to reduce the space between ZnO particles, therefore for example it is possible to make the
elution of Ce from the CeO2 particles deposited in the vicinity of these particles proceed
relatedly slowly and as a result it becomes possible to achieve long term corrosion resistance
25 after coating.
[0048]
[Zn- and Al-Containing Complex Oxide Layer]
In the present invention, the hot stamped body, as explained in relation to FIG. 1, further
contains a Zn- and Al-containing complex oxide layer between the Al-plating layer and the
30 coating containing the ZnO particles and CeO2 particles. FIG. 4 shows a cross-sectional image
obtained by an SEM of a hot stamped body according to one embodiment of the present
invention (amount of deposition of ZnO: 2.49 g/m2). Referring to FIG. 4, it is possible to
confirm a thin layer (Zn- and Al-containing complex oxide layer) 6 between the Al-plating layer
2 and the coating 3 containing the ZnO particles and CeO2 particles. It was learned by later
35 analysis that the Zn- and Al-containing complex oxide layer 6 was a layer comprised of spinel
type complex metal oxides represented by ZnAl2O4 in which some of the elements are replaced
16
or not replaced by Ce. Such a layer is formed at the time of hot stamping at a high temperature,
for example, 850C or a higher high temperature. The coating film adhesion according to the hot
stamped body of the present invention can be improved.
[0049]
5 (Noninclusion of Organic Binder)
According to the present invention, the above coating preferably does not contain a resin or
other organic binder. In a conventional coating including ZnO particles, for example, as the
binder constituent for the ZnO particles, sometimes an organic binder selected from a
polyurethane resin, epoxy resin, acrylic resin, and polyester resin, a silane coupling agent, etc., is
10 used. However, in a coating including ZnO particles, if further including such an organic binder,
under the high temperature at the time of hot stamping, part or all of the carbon forming the
organic binder will burn and form carbon monoxide or carbon dioxide whereby at least part of
the organic binder will be consumed. Furthermore, at the time of such burning, oxygen is liable
to be robbed from part or the majority of the ZnO particles adjoining the organic binder. Here, if
15 ZnO particles are robbed of oxygen and are reduced to metal Zn, since its boiling point is about
907C or relatively low, the metal Zn is liable to partially be consumed at the time of hot
stamping at 900C or a higher high temperature. In such a case, there is a possibility of the
intended form and/or function of the coating including ZnO particles no longer be able to be
sufficiently maintained. Therefore, the coating according to the hot stamped body of the present
20 invention preferably does not contain an organic binder and is more preferably comprised of
only ZnO particles and CeO2 particles.
[0050]
(Structure of Coating)
According to a preferable aspect of the present invention, a coating containing ZnO
25 particles and CeO2 particles has a structure where CeO2 particles are deposited around the ZnO
particles. FIGS. 2A to 2C show images of a cross-section of a coating obtained by observation
using a scan electron microscope (SEM), wherein FIG. 2A shows a cross-sectional SEM image
of a coating containing only ZnO particles and not containing CeO2 particles (Comparative
Example 45), FIG. 2B shows a cross-sectional SEM image of a coating containing ZnO particles
30 and CeO2 particles (CeO2 particles content: 5 mass%), and FIG. 2C shows an enlarged view of
FIG. 2B.
[0051]
Referring to FIGS. 2A and 2B, it will be understood that regardless of the presence or
absence of CeO2 particles, the ZnO particles are present in a state aggregated relatively densely.
35 Further, referring to FIG. 2C, it will be understood that the coating according to the hot stamped
body of the present invention has a structure where CeO2 particles 5 are deposited around the
17
ZnO particles 4 as shown by the arrows. By having such a structure, the ZnO particles can be
made to aggregate relatively densely to reduce the space between the ZnO particles, therefore it
becomes possible to make the elution of Ce from the CeO2 particles deposited around these
particles progress relatively slowly. Therefore, such a structure is extremely advantageous from
the viewpoint of achieving long term corrosion 5 resistance after coating.
[0052]
(Amount of Deposition of ZnO in Coating)
The amount of deposition of ZnO in the coating is preferably 0.60 g/m2 or more and 13.00
g/m2 or less. If the amount of deposition of ZnO in the coating is less than 0.60 g/m2, sometimes
10 the effects obtained due to addition of ZnO particles, for example, the effect of improvement of
the chemical convertability, coating film adhesion, etc., cannot be sufficiently manifested. As a
result, sometimes a drop in the corrosion resistance after coating is invited. On the other hand, if
the amount of deposition of ZnO in the coating becomes more than 13.00 g/m2, the coating
becomes too thick and the space between ZnO particles becomes too small and therefore
15 sometimes elution of Ce to the cathodic reaction region of the exposed part of the steel base
material is inhibited. For example, the amount of deposition of ZnO in the coating may be 0.70
g/m2 or more, 1.00 g/m2 or more, or 1.20 g/m2 or more and/or 10.00 g/m2 or less, 7.00 g/m2 or
less, 6.00 g/m2 or less, 5.00 g/m2 or less, 3.00 g/m2 or less, or 2.00 g/m2 or less. The amount of
deposition of ZnO in the coating is more preferably 1.20 g/m2 or more and 10.00 g/m2 or less,
most preferably 1.20 g/m2 or more and 5.00 g/m2 20 or less.
[0053]
(Content of CeO2 Particles in Coating)
The content of CeO2 particles in the coating is preferably 1.0 mass% or more and 30.0
mass% or less with respect to the total amount of the ZnO particles and CeO2 particles. If the
25 content of CeO2 particles in the coating is less than 1.0 mass% with respect to the total amount
of the ZnO particles and CeO2 particles, sometimes the effect obtained due to addition of CeO2
particles, i.e., the effect of improvement of the corrosion resistance after coating, cannot be
sufficiently manifested. On the other hand, if the content of CeO2 particles in the coating
becomes more than 30.0 mass% with respect to the total amount of the ZnO particles and CeO2
30 particles, the content of ZnO particles becomes smaller, therefore the effect due to the presence
of the ZnO particles, for example, the effect of improvement of the chemical convertability, the
coating film adhesion, etc., can no longer be sufficiently manifested. For example, the content of
CeO2 particles in the coating may be 2.0 mass% or more, 3.0 mass% or more, 4.0 mass% or
more, 5.0 mass% or more, or 6.0 mass% or more and/or 25.0 mass% or less, 20.0 mass% or less,
35 17.0 mass% or less, or 15.0 mass% or less with respect to the total amount of the ZnO particles
and CeO2 particles.
18
[0054]
Furthermore, speaking from the viewpoint of the corrosion resistance after coating, if the
content of CeO2 particles becomes more than 15.0 mass% with respect to the total amount of the
ZnO particles and CeO2 particles, the effect of improvement of the corrosion resistance after
coating is strengthened, b 5 ut on the other hand, elution of Ce is promoted, therefore this
sometimes becomes disadvantageous from the viewpoint of the long term corrosion resistance
after coating. Therefore, the content of CeO2 particles in the coating has to be suitably
determined considering the effect of improvement of the coating film adhesion, etc., due to the
ZnO particles and the effect of improvement of the corrosion resistance after coating due to the
10 CeO2 particles, in particular the effect of improvement of the long term corrosion resistance
after coating. For example, from the viewpoint of reliably maintaining the effect due to the ZnO
particles while realizing to the maximum extent the long term corrosion resistance after coating,
the content of CeO2 particles in the coating is more preferably 2.0 mass% or more and 25.0
mass% or less, most preferably 5.0 mass% or more or 6.0 mass% or more and 15.0 mass% or
15 less, with respect to the total amount of the ZnO particles and CeO2 particles.
[0055]
In the present invention, the amount of deposition of ZnO in the coating and the content of
CeO2 particles with respect to the total amount of the ZnO particles and CeO2 particles are
determined as follows: Specifically, if the aqueous solution or other solution coated on the
20 surface of the Al-plating layer at the time of formation of the coating contains only ZnO particles
and CeO2 particles as coating constituents and the mixing ratio of the same is known, the
amount of deposition of ZnO in the coating and the content of CeO2 particles with respect to the
total amount of the ZnO particles and CeO2 particles are determined from the mixing ratio and
the thickness of the coating formed. On the other hand, if the mixing ratio of the ZnO particles
25 and CeO2 particles in the solution is unknown, the amount of deposition of ZnO in the coating
and the content of CeO2 particles with respect to the total amount of the ZnO particles and
CeO2 particles are determined by analyzing the coating according to the hot stamped body of the
present invention using fluorescent X-ray analysis based on JIS G 3314: 2011. More particularly,
first, fluorescent X-ray analysis is used to measure the amounts of deposition of metal Zn and
30 metal Ce in the coating, then these measurement values are converted to the amounts of
deposition of ZnO and CeO2 to thereby determine the amount of deposition of ZnO and the
amount of deposition of CeO2 in the coating. The content of CeO2 particles is determined from
the ratio of the amount of deposition of CeO2 to the total of these amounts of deposition.
[0056]
35 (Average Particle Size of ZnO Particles and CeO2 Particles)
According to the present invention, preferably the average particle size of the ZnO particles
19
is 0.003 m or more and 8.000 m or less and the average particle size of the CeO2 particles is
3.0% or more and 20.0% or less of the average particle size of the ZnO particles. By controlling
the average particle sizes of the ZnO particles and the CeO2 particles to the above ranges, as
shown in FIG. 2C, it is possible to make the ZnO particles aggregate with each other relatively
densely to make the space between the ZnO particles 5 smaller and form a coating having a
structure where CeO2 particles are deposited around these particles. For this reason, it is
possible to make the elution of Ce proceed relatively slowly and as a result becomes possible to
achieve long term corrosion resistance after coating. For example, the average particle size of the
ZnO particles may be 0.005 m or more, 0.008 m or more, 0.010 m or more, 0.030 m or
10 more, 0.050 m or more, 0.080 m or more, 0.100 m or more, 0.500 m or more, or 0.600 m
or more and/or may be 7.000 m or less, 6.000 m or less, 5.000 m or less, 4.000 m or less,
3.000 m or less, 1.000 m or less, 0.900 m or less, or 0.800 m or less. Similarly, the average
particle size of the CeO2 particles of 4.0% or more, 5.0% or more, 6.0% or more, 8.0% or more,
8.5% or more, 9.0% or more, or 9.5% or more and/or 18.0% or less, 16.0% or less, 14.0% or
15 less, 12.5% or less, 12.0% or less, 11.0% or less, or 10.5% or less of the average particle size of
the ZnO particles. To make the above effect reliable, the average particle size of the ZnO
particles is more preferably 0.050 m or more and 4.000 m or less or 3.000 m or less, most
preferably 0.050 m or more and 0.900 m or less. Similarly, the average particle size of the
CeO2 particles is more preferably more 9.0% or more and 12.0% or less, most preferably 9.5%
20 or more and 10.5% or less, with respect to the average particle size of the ZnO particles.
[0057]
In the present invention, the average particle size of the ZnO particles was determined by
using an SEM or other electron microscope to examine the surface of the coating of the steel
material at a 4 m3 m field (corresponding to 30,000X power) for any two or more locations,
25 selecting 10 or more primary particles (ZnO) for each field and measuring their diameters, and
arithmetically averaging the obtained measurement values. However, for example, if the primary
particles (ZnO) are large, if not possible to measure the diameters of 10 or more primary
particles in a 4 m3 m field, similarly it was determined by examining the surface of the
coating of the hot stamped body at any two or more locations in 12 m9 m fields
30 (corresponding to 10,000X power), selecting 10 or more primary particles (ZnO) for each field
and measuring their diameters, and arithmetically averaging the obtained measurement values.
Furthermore, if not possible to measure the diameters of 10 or more primary particles in 12
m9 m fields, similarly it is determined by examining the surface of the coating of the steel
material at a 36 m27 m field (corresponding to 3,300X power) for any two or more
35 locations, selecting 10 or more primary particles (ZnO) for each field and measuring their
diameters, and arithmetically averaging the obtained measurement values. Regarding the average
20
particle size of the CeO2 particles as well, in the same way as the case of ZnO particles, this was
determined by using a SEM or other electron microscope to examine the surface of the coating
of the hot stamped body at a 4 m3 m field (corresponding to 30,000X power) for any two or
more locations, selecting 10 or more particles deposited around the ZnO particles for each field,
analyzing these particles by an energy dispersive X-5 ray spectrograph (EDS) to confirm the
presence of Ce and thereby identify CeO2 , then measuring their diameters and arithmetically
averaging the obtained measurement values.
[0058]
FIG. 3 is a view schematically showing a method of measurement of the average particle
10 size for ZnO particles and CeO2 particles in the present invention. FIG. 3(a) shows the state of
ZnO particles and CeO2 particles present in a coating before heating relating to hot pressing,
while FIG. 3(b) shows the state of ZnO particles and CeO2 particles present in a coating after
the heating. As shown in FIG. 3(b), due to the heating at a high temperature relating to hot
pressing, at least part of the ZnO particles 4 in the coating melt-bond with each other, but the
15 shapes of the ZnO particles 4 before heating can be sufficiently surmised. In the present
invention, the average particle sizes of the ZnO particles 4 and CeO2 particles 5 present in the
state such as shown in FIG. 3(b) are determined by the method explained above. More
specifically, if, as shown in FIG. 3(b), a particle is spherical or substantially spherical, the
diameter of the particle is simply measured. On the other hand, if a particle is spheroidal or
20 otherwise not spherical, the longest axis of the particle (long axis) and the shortest axis of the
particle perpendicular to the same (short axis) are measured and the arithmetic average is made
the particle size of the particle.
[0059]

25 For example, the hot stamped body of the present invention having the above features can
be produced by a method comprising
forming an Al-plating layer on at least one side of a steel sheet,
coating a surface of the Al-plating layer with an aqueous solution containing ZnO particles
and CeO2 particles, then heating it to form a coating containing ZnO particles and CeO2
30 particles on the Al-plating layer, and
hot pressing the steel sheet having the formed coating thereon. Below, the steps of this
method of production will be explained in detail.
[0060]
[Step for Forming Al-Plating Layer]
35 In the step for forming the Al-plating layer, at least one side of a steel sheet having a
predetermined thickness and composition is formed with Al plating by the Sendzimir process.
21
The steel sheet is not particularly limited, but for example, as explained in relation to the steel
base material, may have a thickness of 0.3 to 2.3 mm and contain, by mass%, C: 0.01 to 0.50%,
Si: 2.00% or less, Mn: 0.01 to 3.50%, P: 0.100% or less, S: 0.050% or less, Al: 0.001 to 0.100%,
and N: 0.020% or less, have a balance of Fe and impurities, and optionally contain, furthermore,
one or more of Ti: 0 to 0.100%, B: 0 to 0.0100%, 5 Cr: 0 to 1.00%, Ni: 0 to 5.00%, Mo: 0 to
2.000%, Cu: 0 to 1.000%, and Ca: 0 to 0.1000%.
[0061]
More specifically, first, the above steel sheet, in particular the cold rolled steel sheet, is
annealed in an N2-H2 mixed gas atmosphere for a predetermined temperature and time, for
10 example, a temperature of 750 to 850C for 10 seconds to 5 minutes, then is cooled down to the
vicinity of the plating bath temperature in a nitrogen atmosphere or other inert atmosphere. Next,
this steel sheet is dipped in an Al plating bath containing 3 mass% or more and 15 mass% or less
of Si at 600 to 750C in temperature for 0.1 to 60 seconds, then is pulled out and immediately
blown with N2 gas or air by the gas wiping method to adjust the amount of deposition of the Al
plating to a predetermined range, for example, 40 to 200 g/m2 15 in range at the two sides. Finally,
the steel sheet is blown with air, etc., to cool it, whereby one side or both sides of the steel sheet
are formed with an Al-plating layer.
[0062]
[Step of Forming Coating]
20 Next, in the step for forming the coating, the Al-plating layer is formed with a coating
containing ZnO particles and CeO2 particles. More specifically, the Al-plating layer is coated by
a bar coater with an aqueous solution containing ZnO particles and CeO2 particles having
average particle sizes within suitable ranges, for example, average particle sizes in the range
explained above in a mixing ratio giving a content of CeO2 particles in the range explained
25 above in the same way. When coating by a bar coater, etc., the wet film thickness is adjusted to
give a predetermined amount of deposition of ZnO, for example, an amount of deposition of
ZnO of 0.60 g/m2 or more and 13.00 g/m2 or less. Finally, the steel sheet is heated at a peak
temperature of 60 to 100C whereby a coating containing ZnO particles and CeO2 particles is
baked on the Al-plating layer.
30 [0063]
[Hot Pressing Step]
Next, the hot stamped body of the present invention is produced by hot stamping the steel
sheet on which the coating containing ZnO particles and CeO2 particles is formed in the hot
pressing step. The above hot pressing can be performed by any method known to a person skilled
35 in the art. While not particularly limited, for example, the steel sheet after the step for forming
the coating can be heated by an approximately 50 to 300C/s rate of temperature rise up to the
22
Ac3 point or more in temperature, generally approximately 850 to 1000C in temperature, then
hot pressed over a predetermined time. Here, with a less than 850C heating temperature, a
sufficient hardness may not be obtained, therefore this is not preferable. Further, if the heating
temperature is more than 1000C, due to the excessive diffusion of Fe from the steel base
material to the Al-plating layer, sometimes the 5 alloying of the Al and Fe progresses too much. In
such a case, a drop in the corrosion resistance after coating is sometimes invited, therefore this is
not preferable. Further, the hardening by the die at the time of hot pressing is not particularly
limited, but, for example, after leaving the heating furnace, the steel sheet is cooled by an
average cooling rate of 30C/s or more until the temperature falls to 400C.
10 [0064]
(Chemical Conversion Treatment and Coating Treatment)
The hot-pressed steel sheet may be chemically converted on the coating containing ZnO
particles and CeO2 particles to form a phosphate coating, then coated by electrodeposition
coating, etc. Due to this, it is possible to improve the adhesion of the coating. The chemical
15 conversion treatment and coating treatment can be performed under any suitable conditions
known to persons skilled in the art.
[0065]

In the present invention, in addition to the above hot-pressed member and method for
20 producing the same, an Al-plated steel sheet suitable for producing the hot stamped body is
provided. The Al-plated steel sheet comprises a steel base material, an Al-plating layer formed
on at least one surface of the steel base material, and a coating formed on the Al-plating layer
and containing ZnO particles and CeO2 particles having an average particle size smaller than an
average particle size of the ZnO particles.
25 [0066]
The Al-plated steel sheet of the present invention corresponds to the hot stamped body
explained above in the state before hot pressing (hot stamping). Therefore, the Al-plated steel
sheet has features similar to the hot stamped body explained previously other than the Zn- and
Al-containing complex oxide layer formed between the Al-plating layer and coating containing
30 ZnO particles and CeO2 particles at the time of hot stamping. Below, these features will be
explained in detail.
[0067]
[Steel Base Material]
The steel base material according to the Al-plated steel sheet of the present invention may
35 be any steel material having a thickness and composition generally used in hot stamped bodies.
As such a steel base material, while not particularly limited to this, a steel material having a 0.3
23
to 2.3 mm thickness and/or containing, by mass%, C: 0.01 to 0.50%, Si: 2.00% or less, Mn: 0.01
to 3.50%, P: 0.100% or less, S: 0.050% or less, Al: 0.001 to 0.100%, N: 0.020% or less and a
balance of Fe and impurities can be mentioned. Below, the constituents contained in the above
steel base material which is preferably applied in the present invention will be explained in more
detail. In the following explanation, the “%” 5 relating to the contents of the constituents means
“mass%” unless otherwise indicated.
[0068]
[C: 0.01 to 0.50%]
Carbon (C) is an element unavoidably contained in steel and/or included so as to secure the
10 targeted mechanical strength. Excessively reducing the C content causes the refining costs to
increase, therefore the C content is preferably 0.01% or more. Further, if the C content is less
than 0.10%, a need arises to include other alloy elements in large amounts so as to secure the
mechanical strength, therefore from the viewpoint of securing the mechanical strength, the C
content is preferably 0.10% or more or 0.20% or more. On the other hand, if the C content is
15 more than 0.50%, the steel material can be made further harder, but becomes brittle and
sometimes hot dip cracks form. Therefore, the C content is preferably 0.50% or less. From the
viewpoint of preventing hot dip cracks, the content is more preferably 0.40% or less or 0.30% or
less.
[0069]
20 [Si: 2.00% or Less]
Si is an element which is added as a deoxidizer and otherwise unavoidably included in the
refining process of steel and an element having the effect of improving strength. The Si content
may be 0%, but from the viewpoint of improving strength, it is preferably 0.01% or more. For
example, the Si content may be 0.05% or more or 0.10% or more. On the other hand, excessive
25 inclusion of Si sometimes causes a drop in ductility in the hot rolling step at the time of
production of steel sheet or as a result causes deterioration of the surface properties. For this
reason, the Si content is preferably 2.00% or less. For example, the Si content may be 1.50% or
less or 1.00% or less. Further, Si is an easily oxidizable element. It forms an oxide film on the
steel sheet surface, therefore if the Si content is more than 0.60%, at the time of hot dip coating,
30 there is a possibility of the wettability falling and nonplating defects arising. Therefore, more
preferably, the Si content is 0.60% or less.
[0070]
[Mn: 0.01 to 3.50%]
Mn is an element which is added as a deoxidizer and otherwise unavoidably included in the
35 refining process of steel, but has the effect of improving the strength and improving the
hardenability and, furthermore, has the effect of suppressing the hot embrittlement due to S.
24
Inclusion of 0.01% or more is preferable. For example, the Mn content may be 0.10% or more or
0.50% or more. On the other hand, if excessively including Mn, sometimes deterioration of the
uniformity of quality due to segregation at the time of casting, excessive hardening of the steel,
and a drop in the ductility at the time of hot and cold working are invited, therefore the Mn
content is preferably 3.50% or less. F 5 or example, the Mn content may be 3.00% or less or 2.00%
or less.
[0071]
[P: 0.100% or Less]
P is an unavoidably contained element, but is also a solution strengthening element and is
10 an element which can improve the strength of a steel material at relatively low cost. However,
excessive inclusion of P sometimes invites a drop in toughness, therefore the P content is
preferably 0.100% or less. For example, the P content may be 0.050% or less or 0.020% or less.
On the other hand, the lower limit of the P content may be 0%, but from the refining limit is
preferably 0.001%. For example, the P content may be 0.003% or more or 0.005% or more.
15 [0072]
[S: 0.050% or Less]
S is also an unavoidably included element. It forms inclusions as MnS which act as starting
points for breakage and sometimes harm ductility and toughness and becomes a cause of inferior
workability. For this reason, the lower the S content, the more preferable. 0.050% or less is more
20 preferable. For example, the S content may be 0.020% or less or 0.010% or less. On the other
hand, the lower limit of the S content may be 0%, but from the production costs is preferably
0.001%. For example, the S content may be 0.002% or more or 0.003% or more.
[0073]
[Al: 0.001 to 0.100%]
25 Al is an element used as a deoxidizer at the time of steelmaking. From the refining limit, the
lower limit of the Al content is preferably 0.001%. For example, the Al content may be 0.005%
or more or 0.010% or more. Further, Al is an element obstructing plateability, therefore the
upper limit of the Al content is preferably 0.100%. For example, the Al content may be 0.080%
or less or 0.050% or less.
30 [0074]
[N: 0.020% or Less]
N is also an unavoidably included element. However, if too great, an increase in the
production costs may be expected, therefore the upper limit of the N content is preferably
0.020%. For example, the N content may be 0.015% or less or 0.010% or less. On the other
35 hand, the lower limit of the N content may be 0%, but from the cost in production is preferably
0.001%. For example, the N content may be 0.002% or more or 0.003% or more.
25
[0075]
The basic chemical composition of the steel base material suitable for use in the present
invention is as explained above. Furthermore, the steel base material may optionally contain one
or more of Ti: 0 to 0.100%, B: 0 to 0.0100%, Cr: 0 to 1.00%, Ni: 0 to 5.00%, Mo: 0 to 2.000%,
Cu: 0 to 1.000%, and Ca: 0 t 5 o 0.1000%. Below, these optional elements will be explained in
detail.
[0076]
[Ti: 0 to 0.100%]
Ti is one of the strengthening elements and an element improving the heat resistance of the
10 Al-based plating layer. If the Ti content is less than 0.005%, the effect of improvement of the
strength or heat resistance cannot be sufficiently obtained, therefore the Ti content is preferably
0.005% or more. For example, the Ti content may be 0.010% or more or 0.015% or more. On the
other hand, if excessively including Ti, for example, carbides and nitrides are formed leading to
softening of the steel material, therefore the Ti content is preferably 0.100% or less. For
15 example, the Ti content may be 0.080% or less or 0.050% or less.
[0077]
[B: 0 to 0.0100%]
B is an element having the effect of acting to improve the strength of the steel material at
the time of hardening. If the B content is less than 0.0003%, such an effect of improving the
20 strength is not sufficiently obtained. On the other hand, if more than 0.0100%, inclusions (for
example, BN, borocarbides, etc.) are formed resulting in embrittlement and the fatigue strength
is liable to be lowered. Therefore, the B content is preferably 0.0003% to 0.0100%. For example,
the B content may be 0.0010% or more or 0.0020% or more and/or 0.0080% or less or 0.0060%
or less.
25 [0078]
[Cr: 0 to 1.00%]
Cr has the effect of suppressing the formation of nitrides formed at the interface of the Albased
plating layer and causing the Al-based plating layer to peel off. Further, Cr is also an
element improving the wear resistance and improving the hardenability. If the Cr content is less
30 than 0.01%, these effects cannot be sufficiently obtained. On the other hand, if the Cr content is
more than 1.00%, not only do the above effects become saturated, but also the production costs
of the steel material rise. Therefore, the Cr content is preferably 0.01 to 1.00%. For example, the
Cr content may be 0.05% or more or 0.10% or more and/or 0.80% or less or 0.50% or less.
[0079]
35 [Ni: 0 to 5.00%]
Ni has the effect of improving the hardenability at the time of hot pressing and also has the
26
effect of raising the corrosion resistance of the steel material itself. If the Ni content is less than
0.01%, these effects cannot be sufficiently obtained, On the other hand, if the Ni content is more
than 5.00%, not only do the above effects become saturated, but also the production costs of the
steel material rise. Therefore, the Ni content is preferably 0.01 to 5.00%. For example, the Ni
content ma 5 y be 0.05% or more or 0.10% or more and/or 3.00% or less or 2.00% or less.
[0080]
[Mo: 0 to 2.000%]
Mo has the effect of improving the hardenability at the time of hot pressing and also has the
effect of raising the corrosion resistance of the steel material itself. If the Mo content is less than
10 0.005%, these effects cannot be sufficiently obtained. On the other hand, if the Mo content is
more than 2.000%, not only do the above effects become saturated, but also the production costs
of the steel material rise. Therefore, the Mo content is preferably 0.005 to 2.000%. For example,
Mo content may be 0.010% or more or 0.100% or more and/or 1.500% or less or 1.000% or less.
[0081]
15 [Cu: 0 to 1.000%]
Cu has the effect of improving the hardenability at the time of hot pressing and also has the
effect of raising the corrosion resistance of the steel material itself. If the Cu content is less than
0.005%, the above effects cannot be sufficiently obtained, On the other hand, if the Cu content is
more than 1.000%, not only do the above effects become saturated, but also the production costs
20 of the steel material rise. Therefore, Cu content is preferably 0.005 to 1.000%. For example, the
Cu content may be 0.010% or more or 0.050% or more and/or 0.500% or less or 0.200% or less.
[0082]
[Ca: 0 to 0.1000%]
Ca is an element for controlling inclusions. If the Ca content is less than 0.0002%, the effect
25 is not sufficiently obtained, therefore the Ca content is preferably 0.0002% or more. For
example, the Ca content may be 0.0010% or more or 0.0020% or more. On the other hand, if the
Ca content is more than 0.1000%, the alloy cost becomes higher, therefore the Ca content is
preferably 0.1000% or less. For example, the Ca content may be 0.0500% or less or 0.0100% or
less.
30 [0083]
Furthermore, the steel base material according to the Al-plated steel sheet of the present
invention may suitably contain other elements in a range not detracting from the effect of the
present invention explained in this Description in addition to or in place of the above optional
elements. For example, W, V, Nb, Sb, and other elements may be suitably included.
35 [0084]
In the steel base material according to the Al-plated steel sheet of the present invention, the
27
balance other than the above constituents consists of Fe and impurities. Here, the impurities in
the steel base material are constituents, etc., entering due to various factors in the production
process such as the ore, scrap or other raw materials when industrially producing the Al-plated
steel sheet according to the present invention.
5 [0085]
[Al-Plating Layer]
According to the present invention, an Al-plating layer is formed on at least one side of the
steel base material, i.e., on one side or both sides of the steel base material. In the Al-plated steel
sheet of the present invention, the “Al-plating layer” means a plating layer in which the chemical
10 composition right after plating mainly consists of Al, more specifically, a plating layer in which
the chemical composition right after plating comprises more than 50 mass% of Al. For example,
the Al content of the Al-plating layer right after plating may be 60 mass% or more, 70 mass% or
more, or 80 mass% or more and 95 mass% or less, 90 mass% or less, or 85 mass% or less.
[0086]
15 The Al-plating layer according to the Al-plated steel sheet of the present invention
preferably contains Si in 3 mass% or more and 15 mass% or less and has a balance of Al and
impurities. For example, the Si content of the Al-plating layer may be 4 mass% or more, 5
mass% or more, or 6 mass% or more and/or 14 mass% or less, 13 mass% or less, or 12 mass% or
less, more preferably 6 mass% or more and 12 mass% or less. In the present invention, the
20 chemical composition of the Al-plating layer can be deemed basically the same as the chemical
composition in the plating bath for forming the Al-plating layer except for unavoidable
impurities entering when forming the Al-plating layer.
[0087]
Furthermore, the Al-plating layer according to the Al-plating steel sheet of the present
25 invention may also suitably contain other elements in a range not obstructing the effect of the
present invention explained in the Description. For example, the Al-plating layer may, in
addition to Si, optionally contain Mg, Ca, Sr, mischmetal, or other elements for improving the
corrosion resistance of the Al-plating layer.
[0088]
30 In the Al-plating layer according to the Al-plated steel sheet of the present invention, the
balance other than the above constituents (i.e., Si, Mg, Ca, Sr, mischmetal, and other elements)
consists of Al and impurities. Here, the “impurities” in the Al-plating layer are raw materials first
and foremost and other constituents, etc., entering due to various factors in the production
process when producing an Al-plating layer. For example, as the impurities in the Al-plating
35 layer, Fe and other steel base material constituents eluting from the steel base material to the
inside of the plating bath may be mentioned. Such a content of Fe is generally 1 mass% or more,
28
more specifically1 to 3 mass% or 1 to 2.5 mass%.
[0089]
[Coating Containing ZnO Particles and CeO2 Particles Having Average Particle Size Smaller
Than Average Particle Size of ZnO Particles]
According to the present invention, the Al-plating 5 layer is formed with a coating containing
ZnO particles and CeO2 particles having an average particle size smaller than the average
particle size of the ZnO particles. The Al-plated steel sheet of the present invention has a
structure the same as the structure of FIG. 1 relating to the hot stamped body other than not
containing the Zn- and Al-containing complex oxide layer. By applying such an Al-plated steel
10 sheet to the production of a hot stamped body, as explained above in relation to the hot stamped
body, even if a defect is formed at the hot stamped body which would reach down to the steel
base material, the Ce constituent, more specifically the Ce4+ ions, is made to be eluted from the
CeO2 particles at the defect part and can form a protective coating at the cathodic reaction
region of the exposed part of the steel base material. More specifically, it is possible to form a
15 protective coating comprised of Ce(OH)4, therefore progression of the cathodic reaction at the
exposed part of the steel base material can be suppressed. As a result, according to the Al-plated
steel sheet of the present invention, it is possible to obtain a hot stamped body remarkably
suppressed in occurrence of coating film blisters, etc., and excellent in corrosion resistance after
coating.
20 [0090]
(Noninclusion of Organic Binder)
The coating according to the Al-plated steel sheet of the present invention preferably does
not contain a resin or other organic binder. In a conventional coating including ZnO particles, for
example, as the binder constituent for the ZnO particles, sometimes an organic binder selected
25 from a polyurethane resin, epoxy resin, acrylic resin, and polyester resin, a silane coupling agent,
etc., is used. However, in a coating including ZnO particles, if further including such an organic
binder, under the high temperature at the time of hot stamping, part or all of the carbon forming
the organic binder will burn and form carbon monoxide or carbon dioxide whereby at least part
of the organic binder will be consumed. Furthermore, at the time of such burning, oxygen is
30 liable to be robbed from part or the majority of the ZnO particles adjoining the organic binder.
Here, if ZnO particles are robbed of oxygen and are reduced to metal Zn, since its boiling point
is about 907C or relatively low, the metal Zn is liable to partially be consumed at the time of hot
stamping at 900C or a higher high temperature. In such a case, there is a possibility of the
intended form and/or function of the coating including ZnO particles no longer be able to be
35 sufficiently maintained. Therefore, the coating according to the Al-plated steel sheet of the
present invention preferably does not contain an organic binder and is more preferably
29
comprised of only ZnO particles and CeO2 particles.
[0091]
(Structure of Coating)
In the Al-plated steel sheet of the present invention, while hot stamping causes part of the
ZnO particles in the coating to melt-5 bond with each other, etc., the basic structure of the coating
does not greatly change before and after the hot stamping. Therefore, according to a preferable
embodiment of the Al-plated steel sheet of the present invention, in the same way as explained in
relation to FIG. 2, the coating containing ZnO particles and CeO2 particles has a structure where
CeO2 particles are deposited around the ZnO particles. By having such a structure, the ZnO
10 particles can be made to aggregate relatively densely to reduce the space between the ZnO
particles, therefore it becomes possible to make the elution of Ce from the CeO2 particles
deposited around these particles progress relatively slowly. Therefore, such a structure is
extremely advantageous from the viewpoint of achieving long term corrosion resistance after
coating.
15 [0092]
(Amount of Deposition of ZnO in Coating)
The amount of deposition of ZnO in the coating according to the Al-plated steel sheet of the
present invention, for reasons similar to explained related to the hot stamped body, is preferably
0.60 g/m2 or more and 13.00 g/m2 or less. For example, the amount of deposition of ZnO in the
coating may be 0.70 g/m2 or more, 1.00 g/m2 or more, or 1.20 g/m2 or more and/or 10.00 g/m2 20
or less, 7.00 g/m2 or less, 6.00 g/m2 or less, 5.00 g/m2 or less, 3.00 g/m2 or less, or 2.00 g/m2 or
less. The amount of deposition of ZnO in the coating is more preferably 1.20 g/m2 or more and
10.00 g/m2 or less, most preferably 1.20 g/m2 or more and 5.00 g/m2 or less.
[0093]
25 (Content of CeO2 Particles in Coating)
The content of CeO2 particles in the coating according to the Al-plated steel sheet of the
present invention is preferably, for reasons similar to explained related to the hot stamped body,
1.0 mass% or more and 30.0 mass% or less with respect to the total amount of the ZnO particles
and CeO2 particles. For example, the content of CeO2 particles in the coating may be 2.0
30 mass% or more, 3.0 mass% or more, 4.0 mass% or more, 5.0 mass% or more, or 6.0 mass% or
more and/or 25.0 mass% or less, 20.0 mass% or less, 17.0 mass% or less, or 15.0 mass% or less
with respect to the total amount of the ZnO particles and CeO2 particles. For example, from the
viewpoint of reliably maintaining the effect due to the ZnO particles while achieving to the
maximum the long term corrosion resistance after coating, the content of CeO2 particles in the
35 coating is more preferably 2.0 mass% or more and 25.0 mass% or less, most preferably 5.0
mass% or more or 6.0 mass% or more and 15.0 mass% or less, with respect to the total amount
30
of the ZnO particles and CeO2 particles.
[0094]
The amount of deposition of ZnO in the coating and the content of CeO2 particles with
respect to the total amount of the ZnO particles and CeO2 particles according to the Al-plated
steel sheet of the present invention are 5 determined in the following way. Specifically, if the
aqueous solution or other solution coated on the surface of the Al-plating layer at the time of
formation of the coating contains only ZnO particles and CeO2 particles as coating constituents
and the mixing ratio of the same is known, the amount of deposition of ZnO in the coating and
the content of CeO2 particles with respect to the total amount of the ZnO particles and CeO2
10 particles are determined from the mixing ratio and the thickness of the coating formed. On the
other hand, if the mixing ratio of the ZnO particles and CeO2 particles in the solution is
unknown, the amount of deposition of ZnO in the coating and the content of CeO2 particles with
respect to the total amount of the ZnO particles and CeO2 particles are determined by analyzing
the coating according to the Al-plated steel sheet of the present invention using fluorescent X-ray
15 analysis based on JIS G 3314: 2011. More particularly, first, fluorescent X-ray analysis is used to
measure the amounts of deposition of metal Zn and metal Ce in the coating, then these
measurement values are converted to the amounts of deposition of ZnO and CeO2 to thereby
determine the amount of deposition of ZnO and the amount of deposition of CeO2 in the
coating. The content of CeO2 particles is determined from the ratio of the amount of deposition
20 of CeO2 to the total of these amounts of deposition.
[0095]
(Average Particle Sizes of ZnO Particles and CeO2 Particles)
In the Al-plated steel sheet of the present invention, due to reasons similar to those
explained in relation to the hot stamped body, preferably the average particle size of the ZnO
25 particles is 0.003 m or more and 8.000 m or less and the average particle size of the CeO2
particles is 3.0% or more and 20.0% or less of the average particle size of the ZnO particles. For
example, the average particle size of the ZnO particles may be 0.005 m or more, 0.008 m or
more, 0.010 m or more, 0.030 m or more, 0.050 m or more, 0.080 m or more, 0.100 m or
more, 0.500 m or more, or 0.600 m or more and/or 7.000 m or less, 6.000 m or less, 5.000
30 m or less, 4.000 m or less, 3.000 m or less, 1.000 m or less, 0.900 m or less, or 0.800 m
or less. Similarly, the average particle size of the CeO2 particles may be 4.0% or more, 5.0% or
more, 6.0% or more, 8.0% or more, 8.5% or more, 9.0% or more, or 9.5% or more and/or 18.0%
or less, 16.0% or less, 14.0% or less, 12.5% or less, 12.0% or less, 11.0% or less, or 10.5% or
less of the average particle size of the ZnO particles. The average particle size of the ZnO
35 particles is more preferably 0.050 m or more and 4.000 m or less or 3.000 m or less, most
preferably 0.050 m or more and 0.900 m or less. Similarly, the average particle size of the
31
CeO2 particles is more preferably 9.0% or more and 12.0% or less, most preferably 9.5% or
more and 10.5% or less, of the average particle size of the ZnO particles.
[0096]
In the present invention, the average particle size of the ZnO particles was determined by
using a scan electron microscope (SEM) or othe 5 r electron microscope to examine the surface of
the coating of the Al-plated steel material in a 4 m3 m field (corresponding to 30,000X
power) for any two or more locations, selecting 10 or more primary particles (ZnO) for each
field and measuring their diameters, and arithmetically averaging the obtained measurement
values. However, for example, if the primary particles (ZnO) are large, if not possible to measure
10 the diameters of 10 or more primary particles in a 4 m3 m field, similarly it was determined
by examining the surface of the coating of the hot stamped body in a 12 m9 m field
(corresponding to 10,000X power) for any two or more locations, selecting 10 or more primary
particles (ZnO) for each field and measuring their diameters, and arithmetically averaging the
obtained measurement values. Furthermore, if not possible to measure the diameters of 10 or
15 more primary particles in a 12 m9 m fields, similarly it is determined by examining the
surface of the coating of the steel material at a 36 m27 m field (corresponding to 3,300X
power) for any two or more locations, selecting 10 or more primary particles (ZnO) for each
field and measuring their diameters, and arithmetically averaging the obtained measurement
values. Regarding the average particle size of the CeO2 particles as well, in the same way as the
20 case of ZnO particles, this was determined by using a SEM or other electron microscope to
examine the surface of the coating of the hot stamped body at a 4 m3 m field (corresponding
to 30,000X power) for any two or more locations, selecting 10 or more particles deposited
around the ZnO particles for each field, analyzing these particles by an energy dispersive X-ray
spectrograph (EDS) to confirm the presence of Ce and thereby identify CeO2 , then measuring
25 their diameters and arithmetically averaging the obtained measurement values.
[0097]
In the Al-plated steel sheet of the present invention, heating under a high temperature
relating to hot pressing is not performed, therefore part of the ZnO particles in the coating do not
melt-bond with each other. For this reason, the ZnO particles and CeO2 particles are present in
30 the coating in the state such as shown in FIG. 3(a). Therefore, in the Al-plated steel sheet of the
present invention, at the time of measurement of the average particle size of the ZnO particles
and the CeO2 particles by the method explained above, if, as shown in FIG. 3(a), a particle is
spherical or substantially spherical, the diameter of the particle is simply measured. On the other
hand, if a particle is spheroidal or otherwise not spherical, the longest axis of the particle (long
35 axis) and the shortest axis of the particle perpendicular to the same (short axis) are measured and
the arithmetic average is made the particle size of the particle.
32
[0098]
Below, examples will be used to explain the present invention in more detail, but the
present invention is not limited to these examples in any way.
5 EXAMPLES
[0099]
In the following examples, steel materials for the hot stamped body according to the present
invention were produced under various conditions and were examined for their corrosion
resistance after coating.
10 [0100]
First, in each example, cold-rolled steel sheet (sheet thickness 1.4 mm) having a chemical
composition comprising, by mass%, C: 0.22%, Si: 0.12%, Mn: 1.25%, P: 0.010%, S: 0.005%,
Al: 0.040%, N: 0.001%, Ti: 0.020%, B: 0.0030%, and a balance of Fe and impurities was formed
on its two sides with Al-plating layers by the Sendzimir process. More specifically, first, the
15 above cold-rolled steel sheet was annealed in an N2 -H2 mixed gas (H2 4%, N2 balance)
atmosphere at 800C for 1 minute, then was cooled down to the vicinity of the plating bath
temperature in a nitrogen atmosphere. Next, this steel sheet was dipped in an Al plating bath
containing 9 mass% of Si at a temperature of 670C for 3 seconds, then was pulled out and
immediately blown with N2 gas by the gas wiping method to adjust the amounts of deposition of
Al plating on the two sides to 160 g/m2 (single side 80 g/m220 ). Next, the steel sheet was blown
with air to cool it and thereby form an Al-plating layer on the two sides of the steel sheet.
[0101]
Next, the cooled Al-plating layer was coated with an aqueous solution containing ZnO
particles and CeO2 particles in the mixing ratio shown in Table 1 by a bar coater. The wet film
25 thickness was adjusted to give the amount of deposition of ZnO shown in Table 1. Next, the steel
sheet was heated at a peak temperature of 80C to thereby bake a coating containing ZnO
particles and CeO2 particles on the Al-plating layer. This was cooled, then the obtained steel
sheet was cut to 120 mm200 mm in size. The amounts of deposition and mixing ratio of ZnO
and CeO2 shown in Table 1 were those calculated from the mixing ratio of the ZnO particles and
30 CeO2 particles in the aqueous solution coated on the surface of the Al-plating layer and the
thickness of the coating, but these values were equivalent to the measurement values determined
for the steel material corresponding to the finally obtained hot stamped body by fluorescent Xray
analysis based on JIS G 3314: 2011.
[0102]
35 [Table 1]
Table 1
33
No.
Coating
Coating
film blister
width
(mm)
Evaluation Remarks
Amount of
deposition (g/m
2
)
Mixing ratio
(mass％)
Average particle size
(m)
B/A (％)
ZnO CeO2 ZnO CeO2
A B
ZnO CeO2
1 0.37 0.041 90 10 0.8 0.08 10.0 5.10 Fair Ex.
2 0.58 0.064 90 10 0.8 0.08 10.0 5.03 Fair Ex.
3 0.62 0.069 90 10 0.8 0.08 10.0 5.00 Fair Ex.
4 1.00 0.111 90 10 0.8 0.08 10.0 4.90 Fair Ex.
5 1.24 0.006 99.5 0.5 0.8 0.08 10.0 5.10 Fair Ex.
6 1.24 0.010 99.2 0.8 0.8 0.08 10.0 4.95 Fair Ex.
7 1.24 0.013 99 1 0.8 0.08 10.0 4.90 Fair Ex.
8 1.24 0.025 98 2 0.8 0.08 10.0 4.80 Good Ex.
9 1.24 0.052 96 4 0.8 0.08 10.0 4.10 Good Ex.
10 1.24 0.079 94 6 0.8 0.08 10.0 3.60 Very good Ex.
11 1.24 0.108 92 8 0.8 0.08 10.0 3.40 Very good Ex.
12 1.24 0.138 90 10 0.8 0.08 10.0 3.20 Very good Ex.
13 1.24 0.170 88 12 0.8 0.08 10.0 3.40 Very good Ex.
14 1.24 0.220 85 15 0.8 0.08 10.0 3.70 Very good Ex.
15 1.24 0.311 80 20 0.8 0.08 10.0 4.00 Good Ex.
16 1.24 0.415 75 25 0.8 0.08 10.0 4.30 Good Ex.
17 1.24 0.533 70 30 0.8 0.08 10.0 4.90 Fair Ex.
18 1.24 0.670 65 35 0.8 0.08 10.0 5.10 Fair Ex.
19 1.24 0.584 63 32 0.8 0.08 10.0 4.95 Fair Ex.
20 6.22 0.692 90 10 0.8 0.08 10.0 4.20 Good Ex.
21 9.96 1.106 90 10 0.8 0.08 10.0 4.50 Good Ex.
22 12.4 1.383 90 10 0.8 0.08 10.0 5.00 Fair Ex.
23 13.2 1.467 90 10 0.8 0.08 10.0 5.07 Fair Ex.
24 16.2 1.798 90 10 0.8 0.08 10.0 5.20 Fair Ex.
25 1.24 0.138 90 10 0.08 0.0056 7.0 5.10 Fair Ex.
26 1.24 0.138 90 10 0.8 0.056 7.0 5.20 Fair Ex.
27 1.24 0.138 90 10 0.8 0.062 7.8 5.10 Fair Ex.
28 1.24 0.138 90 10 0.8 0.064 8.0 5.05 Fair Ex.
29 1.24 0.138 90 10 0.8 0.068 8.5 5.00 Fair Ex.
30 1.24 0.138 90 10 0.8 0.072 9.0 4.30 Good Ex.
31 1.24 0.138 90 10 0.08 0.0072 9.0 4.80 Good Ex.
32 1.24 0.138 90 10 0.8 0.088 11.0 4.10 Good Ex.
33 1.24 0.138 90 10 0.8 0.1 12.5 5.00 Fair Ex.
34 1.24 0.138 90 10 0.8 0.104 13.0 5.10 Fair Ex.
35 1.24 0.138 90 10 0.08 0.0088 11.0 4.50 Good Ex.
36 1.24 0.138 90 10 0.08 0.0104 13.0 5.20 Fair Ex.
37 1.24 0.138 90 10 0.005 0.0005 10.0 5.10 Fair Ex.
38 1.24 0.138 90 10 0.007 0.0007 10.0 5.06 Fair Ex.
39 1.24 0.138 90 10 0.01 0.001 10.0 5.00 Fair Ex.
40 1.24 0.138 90 10 1 0.1 10.0 4.00 Good Ex.
41 1.24 0.138 90 10 3 0.3 10.0 4.40 Good Ex.
42 1.24 0.138 90 10 5 0.5 10.0 5.00 Fair Ex.
43 1.24 0.138 90 10 6 0.6 10.0 5.05 Fair Ex.
44 1.24 0.138 90 10 7 0.7 10.0 5.10 Fair Ex.
45 1.24 0.000 100 0 0.08 - - 5.30 Poor Comp. ex.
[0103]
Next, the steel sheet was loaded into a furnace simulating hot stamping and was set on a 70
mm70 mm SiC table with the evaluated surface facing upward. Next, this was set on a 50
mm50 mm70 mm SUS304 block heated to 900C and was heated in that state for 1 minute.
Finally, the steel sheet was taken out from the furnace, then 5 was immediately clamped by a
34
stainless steel die and rapidly cooled by an approximately 150C/s cooling rate to obtain a steel
material corresponding to the hot stamped body according to the present invention. Each
obtained steel material was measured for the average particle sizes of the ZnO particles and the
CeO2 particles in the coating and further were tested for the corrosion resistance after coating
5 explained below.
[0104]
[Measurement of Average Particle Sizes of ZnO Particles and CeO2 Particles]
The average particle size of the ZnO particles was determined by using an SEM to examine
the surface of the coating of the steel material at a 4 m3 m field (corresponding to 30,000X
10 power) for any two locations, selecting 10 primary particles (ZnO) for each field and measuring
their diameters, and arithmetically averaging the obtained measurement values. Further, if not
possible to measure the diameters of 10 or more primary particles in a 4 m3 m field,
similarly it was determined by examining the surface of the coating of the steel material at a 12
m9 m field (corresponding to 10,000X power) for any two or more locations, selecting 10
15 or more primary particles (ZnO) for each field and measuring their diameters, and arithmetically
averaging the obtained measurement values. Furthermore, if not possible to measure the
diameters of 10 or more primary particles in a 12 m9 m field, similarly it is determined by
examining the surface of the coating of the steel material at a 36 m27 m field (corresponding
to 3,300X power) for any two or more locations, selecting 10 or more primary particles (ZnO)
20 for each field and measuring their diameters, and arithmetically averaging the obtained
measurement values. On the other hand, the average particle size of the CeO2 particles was
determined by using a scan electron microscope with an energy dispersive X-ray spectrometer
(SEM-EDS) to examine the surface of the coating of the steel material at a 4 m3 m field
(corresponding to 30,000X power) for any two or more locations, selecting 10 particles
25 deposited around the ZnO particles for each field, analyzing these particles by EDS to confirm
the presence of Ce and thereby identify CeO2 , then measuring their diameters and arithmetically
averaging the obtained measurement values. The Al-plated steel sheet before heating at 900C
was similarly measured for the average particle sizes of the ZnO particles and the CeO2
particles, but these values were equal to the average particle sizes of the ZnO particles and the
30 CeO2 particles after heating at 900C and rapid cooling by about 150C/s shown in Table 1.
[0105]
[Test of Corrosion Resistance After Coating]
The end parts of each obtained steel material were cut off to obtain from the center part a
sample of 70 mm150 mm size. The sample was chemically converted by a chemical conversion
35 solution (PB-SX35 made by Nihon Parkerizing Co., Ltd.), then was coated with an
electrodeposition coating (Powernix 110 made by Nippon Paint) to a film thickness of 15 m
35
and baked at 170C. Next, the coating film was given a cut (defect) by an acrylic cutter and it
was confirmed the cut reached the steel base. The steel material was measured for the width of
the coating film blister (maximum value at one side) from the cut part after 122 cycles in the
SAE J2334 test. The results are shown in Table 1. Here, the smaller the value of the “coating
blister width” in Table 1, the better the corrosion 5 resistance after coating of the steel material that
is meant. In Table 1, as a comparative example, the test results of a steel material provided with a
coating not containing CeO2 particles but comprised of only ZnO particles are also shown
(Comparative Example 45 in Table 1). Further, the coating blister width was scored as follows:
Very good: coating blister width less than 4.00 mm
10 Good: coating blister width 4.00 mm or more and less than 4.90 mm
Fair: coating blister width 4.90 mm or more and less than 5.25 mm
Poor: coating blister width 5.25 mm or more
[0106]
Referring to Table 1, in the steel material of Comparative Example 45 provided with a
15 coating not containing CeO2 particles but comprised of only ZnO particles, the coating blister
width was 5.30 mm, while in the steel materials according to all of the other examples, smaller
coating blister widths, specifically less than 5.25 mm coating blister widths, can be achieved. For
this reason, by including in the coating CeO2 particles having an average particle size smaller
than the average particle size of the ZnO particles, it was possible to improve the corrosion
20 resistance after coating. Further, as clear from the results of Table 1, by suitably controlling the
amount of deposition of ZnO in the coating, the CeO2 particle content in the coating, and the
average particle sizes of the ZnO particles and the CeO2 particles, it was possible to further
reduce the coating blister width.
[0107]
25 More specifically, by controlling the amount of deposition of ZnO in the coating to 1.20
g/m2 or more and 10.00 g/m2 or less, the content of CeO2 particles in the coating to 2.0 mass%
or more and 25.0 mass% or less, the average particle size of the ZnO particles to 0.050 m or
more and 4.000 m or less, and the average particle size of the CeO2 particles to 9.0% or more
and 12.0% or less of the average particle size of the ZnO particles, it was possible to reduce the
30 coating blister width to less than 4.90 mm (see Examples 8 to 16, 20, 21, 30 to 32, 35, 40, and 41
evaluated as “very good” and “good”). In particular, in the steel materials of Examples 10 to 14,
by controlling the amount of deposition of ZnO in the coating to 1.20 g/m2 or more and 5.00
g/m2 or less, the content of CeO2 particles in the coating to 5.0 mass% or more and 15.0 mass%
or less, the average particle size of the ZnO particles to 0.050 m or more and 0.900 m or less,
35 and the average particle size of the CeO2 particles to 9.5% or more and 10.5% or less of the
average particle size of the ZnO particles, it was possible to reduce the coating blister width to
36
3.20 to 3.70 mm (corresponding to evaluation as “very good”). It is believed such results are
related to the structure of the coating shown in FIGS. 2B and 2C, i.e., the structure where CeO2
particles are deposited around ZnO particles. Further, while not shown in Table 1, from the
image of the cross-section observed by an SEM and other analysis, it was confirmed that in the
steel materials of all of the examples, there was a Zn- 5 and Al-containing complex oxide layer
present between the Al-plating layer and the coating containing ZnO particles and CeO2
particles and that this was a layer comprised of spinel type complex metal oxides represented by
ZnAl2O4 in which some of the elements are replaced or not replaced by Ce.
10 REFERENCE SIGNS LIST
[0108]
1. steel base material
2. Al-plating layer
3. coating containing ZnO particles and CeO2 particles
15 4. ZnO particles
5. CeO2 particles
6. Zn- and Al-containing complex oxide layer
10. hot stamped body

CLAIMS
[Claim 1]
A hot stamped body comprising
5 a steel base material,
an Al-plating layer formed on at least one surface of the steel base material,
a coating formed on the Al-plating layer and containing ZnO particles and CeO2 particles
having an average particle size smaller than an average particle size of the ZnO particles, and
a Zn- and Al-containing complex oxide layer formed between the Al-plating layer and the
10 coating.
[Claim 2]
The hot stamped body according to claim 1, wherein the coating does not contain an
organic binder.
15
[Claim 3]
The hot stamped body according to claim 1 or 2, wherein the coating has a structure in
which the CeO2 particles are deposited around the ZnO particles.
20 [Claim 4]
The hot stamped body according to any one of claims 1 to 3, wherein an amount of
deposition of ZnO in the coating is 0.60 g/m2 or more and 13.00 g/m2 or less.
[Claim 5]
25 The hot stamped body according to claim 4, wherein the amount of deposition of ZnO in the
coating is 1.20 g/m2 or more and 10.00 g/m2 or less.
[Claim 6]
The hot stamped body according to any one of claims 1 to 5, wherein the coating contains
30 the CeO2 particles in 1.0 mass% or more and 30.0 mass% or less with respect to a total amount
of the ZnO particles and the CeO2 particles.
[Claim 7]
The hot stamped body according to claim 6, wherein the coating contains the CeO2
35 particles in 2.0 mass% or more and 25.0 mass% or less with respect to the total amount of the
ZnO particles and the CeO2 particles.
38
[Claim 8]
The hot stamped body according to any one of claims 1 to 7, wherein an average particle
size of the ZnO particles is 0.003 m or more and 8.000 m or less and an average particle size
of the CeO2 particles is 3.0% 5 or more and 20.0% or less of the average particle size of the ZnO
particles.
[Claim 9]
The hot stamped body according to claim 8, wherein the average particle size of the ZnO
10 particles is 0.010 m or more and 5.000 m or less and the average particle size of the CeO2
particles is 8.0% or more and 12.5% or less of the average particle size of the ZnO particles.
[Claim 10]
The hot stamped body according to claim 9, wherein the average particle size of the ZnO
15 particles is 0.050 m or more and 4.000 m or less and the average particle size of the CeO2
particles is 9.0% or more and 12.0% or less of the average particle size of the ZnO particles.
[Claim 11]
The hot stamped body according to any one of claims 1 to 10, wherein the steel base
20 material comprises, by mass%,
C: 0.01 to 0.50%,
Si: 2.00% or less,
Mn: 0.01 to 3.50%,
P: 0.100% or less,
25 S: 0.050% or less,
Al: 0.001 to 0.100%,
N: 0.020% or less,
Ti: 0 to 0.100%,
B: 0 to 0.0100%,
30 Cr: 0 to 1.00%,
Ni: 0 to 5.00%,
Mo: 0 to 2.000%,
Cu: 0 to 1.000%,
Ca: 0 to 0.1000%, and
35 a balance of Fe and impurities.
39
[Claim 12]
The hot stamped body according to any one of claims 1 to 11, wherein the Al-plating layer
comprises Si and a balance of Al, Fe and impurities.
5 [Claim 13]
A method for producing a hot stamped body according to any one of claims 1 to 12,
comprising
forming an Al-plating layer on at least one side of a steel sheet,
coating a surface of the Al-plating layer with an aqueous solution containing ZnO particles
10 and CeO2 particles, then heating it to form a coating containing ZnO particles and CeO2
particles on the Al-plating layer, and
hot pressing the steel sheet having the formed coating thereon.
[Claim 14]
15 An Al-plated steel sheet comprising
a steel base material,
an Al-plating layer formed on at least one surface of the steel base material, and
a coating formed on the Al-plating layer and containing ZnO particles and CeO2 particles
having an average particle size smaller than an average particle size of the ZnO particles.
20
[Claim 15]
The Al-plated steel sheet according to claim 14, wherein the coating does not contain an
organic binder.
25 [Claim 16]
The Al-plated steel sheet according to claim 14 or 15, wherein the coating has a structure in
which the CeO2 particles are deposited around the ZnO particles.